A tire having a decreased pressure has a decreased outer diameter (tire effective rolling radius) compared with those of tires having a specified internal pressure (normal air pressure) and thus has an increased wheel speed (rotation angular velocity) compared with those of other normal tires. An apparatus for sensing, based on this principle, a decrease in tire air pressure by the relative comparison among the rotation angular velocities of tires during running has been known as an apparatus for detecting a decrease in tire air pressure based on the dynamic loaded radius (DLR) method (see Patent Literature 1 for example).
In the detection apparatus as described above, determination values DEL1 to DEL3 as shown by the following formula (1) for example are used to determine a decrease in tire air pressure.
                                                                                          DEL                  ⁢                                                                          ⁢                  1                                =                                                      [                                                                  {                                                                              (                                                                                          F                                ⁢                                                                                                                                  ⁢                                1                                                            +                                                              F                                ⁢                                                                                                                                  ⁢                                4                                                                                      )                                                    /                                                      (                                                                                          F                                ⁢                                                                                                                                  ⁢                                2                                                            +                                                              F                                ⁢                                                                                                                                  ⁢                                3                                                                                      )                                                                          }                                            -                      1                                        ]                                    ×                  100                  ⁢                                                                          ⁢                                      (                    %                    )                                                                                                                                            DEL                  ⁢                                                                          ⁢                  2                                =                                                      [                                                                  {                                                                              (                                                                                          F                                ⁢                                                                                                                                  ⁢                                1                                                            +                                                              F                                ⁢                                                                                                                                  ⁢                                2                                                                                      )                                                    /                                                      (                                                                                          F                                ⁢                                                                                                                                  ⁢                                3                                                            +                                                              F                                ⁢                                                                                                                                  ⁢                                4                                                                                      )                                                                          }                                            -                      1                                        ]                                    ×                  100                  ⁢                                                                          ⁢                                      (                    %                    )                                                                                                                                            DEL                  ⁢                                                                          ⁢                  3                                =                                                      [                                                                  {                                                                              (                                                                                          F                                ⁢                                                                                                                                  ⁢                                1                                                            +                                                              F                                ⁢                                                                                                                                  ⁢                                3                                                                                      )                                                    /                                                      (                                                                                          F                                ⁢                                                                                                                                  ⁢                                2                                                            +                                                              F                                ⁢                                                                                                                                  ⁢                                4                                                                                      )                                                                          }                                            -                      1                                        ]                                    ×                  100                  ⁢                                                                          ⁢                                      (                    %                    )                                                                                      }                            (        1        )            
In the formula, F1 to F4 represent the wheel speeds of a left front wheel tire, a right front wheel tire, a left rear wheel tire, and a right rear wheel tire, respectively.
In the DLR-type apparatus for detecting a decrease in tire air pressure, an important technique is to estimate the vehicle mass. The reason is that a tire dynamic loaded radius changes depending on a tire air pressure and also depending on the load applied to a tire. Specifically, in the case of DLR method to sense a decrease in tire air pressure based on a learnt value at a normal internal pressure as a reference, it is impossible to determine, if the vehicle mass during an actual running changes from the one at a normal internal pressure (at learning), whether a change of the determination value DEL is caused due to a load change or is caused due to a decreased pressure, thus failing to correctly sense a decrease in tire air pressure.
To solve this, there have been suggested various techniques to consider (or correct) a decrease in the tire outer diameter due to the load to detect a decrease in tire air pressure. For example, in Patent Literature 2 and Patent Literature 3 according to patent applications by the present applicant, a tire driving stiffness being proportional to (wheel load/internal pressure) is used to estimate the vehicle mass based on the following formula (2). This estimated vehicle mass is used to quantitatively correct a decrease in the dynamic loaded radius due to a load.m(α+g sin(θ))+AV2=Fx=T/R  (2)
In the formula, m represents a vehicle mass, α represents vehicle acceleration, g represents gravity acceleration, θ represents a road surface inclination angle, A represents an aerodynamic drag, V represents a vehicle speed, Fx represents a driving force, T represents a vehicle total axle shaft torque, and R represents a tire loaded radius.
The techniques disclosed in Patent Literatures 2 and 3 require not only the speed, acceleration, and axle shaft torque showing vehicle information but also information regarding the inclination angle of the road surface on which the vehicle is running. These pieces of information have a remarkable influence on the accuracy at which the vehicle mass is estimated.
The technique disclosed in Patent Literature 2 uses information obtained from a GPS apparatus. However, a vehicle not including such a GPS apparatus cannot obtain at least information regarding the road surface inclination angle θ, thus failing to estimate the vehicle mass accurately.
To solve this, the technique disclosed in Patent Literature 3 suggests a method by which the vehicle mass can be estimated even without the GPS apparatus. Specifically, based on an assumption that there is a small possibility where the road surface inclination changes at a short time interval, the vehicle mass m is estimated by subjecting data corresponding to a predetermined short time to the iterative least squares technique with regard to the relation of the above formula (2) with presuming mg sin (θ) related to a slope inclination as a constant term.
Furthermore, an abnormal value even during a short time is rejected to thereby minimize the influence by a case where a slope inclination changes. Then, the remaining data group is subjected to the iterative least squares technique with regard to the vehicle acceleration a with presuming Fx−(mg sin(θ)+AV2) as a dependent variable to thereby calculate a corrected vehicle mass.